I'm not a neurologist by any means. However, my mother and I have had conversations about things like this. My stepfather's mother developed dementia (Alzheimers, Parkinson, whatever you want to call it) several years ago. She was in and out of different housing arrangements because she would be so disruptive. Her husband didn't exactly age gracefully either, but his body was in worse shape than his mind. His wife however was verbally abusive of him.
It finally got to the point where her dementia got so bad she simply couldn't function at all in a normal living situaton. She had to be sent to a nursing home. My mother talked to me later about helping move out all her things. She said she found some of her old journals and thumbed through them. She was struck by how angry and bitter she would get over day to day things and how she just seemed to rave about the same things over and over again. She told me she believed the reason my stepdads mother developed her neurodegenerative condition was because she held onto things that made her mad. She took them and kept them in her and that, my mother suggested, began to break her down mentally over time. Sort of like when you don't eat healthy food your immune system isn't very strong so you get sick more easily.
In addition, once my stepdads mother and father were seperated, his father immediately started to seem much more mentally sharp and focused.
I don't know if what your doctor says is true. Odds are he's not certain either. Neurodegenerative diseases are one of those things that no one is certain of the cause. There are studies suggesting that living close to a highway increases a child's risk of developing autism. No one really knows. However my feeling is that if someone develops PTSD but never really deals with it there chances of developing dementia or something like that would increase. If the sufferer takes steps to reconcile the trauma and reduce the stress, I doubt it would effect them in that way.
